Electrochemical cell system and method of operation

ABSTRACT

In one embodiment, the electrochemical system comprises an electrochemical cell and hydrogen storage in fluid communication with the hydrogen electrode, the hydrogen storage comprising at least one of carbon nanotubes and carbon nanofibers. In one embodiment, the method for operating an electrochemical cell system, comprises introducing water to an oxygen electrode and electrolyzing the water to form oxygen, hydrogen ions and electrons, wherein the hydrogen ions migrate to a hydrogen electrode. The hydrogen ions can then be reacted with the electrons to form hydrogen gas that is stored in at least one of carbon nanotubes and carbon nanofibers.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patent application Ser. No. 10/224,070, filed Aug. 20, 2002, which is a continuation of U.S. patent application Ser. No. 09/736,964, filed Dec. 14, 2000, which claims the benefit of the Provisional Application Serial No. 60/171,122 filed Dec. 16, 1999, and a continuation-in-part of U.S. patent application Ser. No. 09/745,614, filed Dec. 22, 2000, which claims the benefit of the Provisional Application Serial No. 60/171,369 filed Dec. 22, 1999; all of which are incorporated herein by reference in their entireties.

BACKGROUND

[0002] Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. An electrolysis cell functions as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gases, and functions as a fuel cell by electrochemically reacting hydrogen with oxygen to generate electricity.

[0003] Referring to FIG. 1, a partial section of a typical proton exchange membrane fuel cell 10 is detailed. In fuel cell 10, hydrogen gas 12 and reactant water 14 are introduced to a hydrogen electrode (anode) 16, while oxygen gas 18 is introduced to an oxygen electrode (cathode) 20. The hydrogen gas 12 for fuel cell operation can originate from a pure hydrogen source, methanol or other hydrogen source. Hydrogen gas electrochemically reacts at anode 16 to produce hydrogen ions (protons) and electrons, wherein the electrons flow of from anode 16 through an electrically connected external load 21, and the protons migrate through a membrane 22 to cathode 20. At cathode 20, the protons and electrons react with the oxygen gas to form resultant water 14′, which additionally includes any reactant water 14 dragged through membrane 22 to cathode 20. The electrical potential across anode 16 and cathode 20 can be exploited to power an external load.

[0004] The same configuration as is depicted in FIG. 1 for a fuel cell is conventionally employed for electrolysis cells. In a typical anode feed water electrolysis cell (not shown), process water is fed into a cell on the side of the oxygen electrode (in an electrolytic cell, the anode) to form oxygen gas, electrons, and protons. The electrolytic reaction is facilitated by the positive terminal of a power source electrically connected to the anode and the negative terminal of the power source connected to a hydrogen electrode (in an electrolytic cell, the cathode). The oxygen gas and a portion of the process water exit the cell, while protons and water migrate across the proton exchange membrane to the cathode where hydrogen gas is formed. In a cathode feed electrolysis cell (not shown), process water is fed on the hydrogen electrode, and a portion of the water migrates from the cathode across the membrane to the anode where protons and oxygen gas are formed. A portion of the process water exits the cell at the cathode side without passing through the membrane. The protons migrate across the membrane to the cathode where hydrogen gas is formed.

[0005] The typical electrochemical cell includes a one or more individual cells arranged in a stack, with the working fluid directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode. In certain conventional arrangements, the anode, cathode, or both are gas diffusion electrodes that facilitate gas diffusion to the membrane. Each cathode/membrane/anode assembly (hereinafter “membrane electrode assembly”, or “MEA”) is typically supported on both sides by flow fields comprising screen packs or bipolar plates. Such flow fields facilitate fluid movement and membrane hydration and provide mechanical support for the MEA. Since a differential pressure often, exists in the cells, compression pads or other compression means are often employed to maintain uniform compression in the cell active area, i.e., the electrodes, thereby maintaining intimate contact between flow fields and cell electrodes over long time periods.

[0006] In certain arrangements, the electrochemical cells can be employed to both convert electricity into hydrogen, and hydrogen back into electricity as needed. Such systems are commonly referred to as regenerative fuel cell systems.

[0007] The typical electrochemical cell system includes a number of individual cells arranged in a stack, with the working fluid directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode. In certain conventional arrangements, the anode, cathode, or both are gas diffusion electrodes that facilitate gas diffusion to the membrane. Each cathode/membrane/anode assembly (hereinafter “membrane electrode assembly”, or “MEA”) is typically supported on both sides by flow fields comprising screen packs or bipolar plates. Such flow fields facilitate fluid movement and membrane hydration and provide mechanical support for the MEA.

[0008] While existing electrochemical cell systems are suitable for their intended purposes, there still remains a need for improvements, particularly regarding hydrogen storage and reduced start up times.

SUMMARY OF THE INVENTION

[0009] Disclosed herein are methods for operating an electrochemical system and electrochemical systems. In one embodiment, the method for operating an electrochemical system, comprises: introducing water to an oxygen electrode; electrolyzing the water to form oxygen, hydrogen ions and electrons, wherein the hydrogen ions migrate to a hydrogen electrode; reacting the hydrogen ions with the electrons to form hydrogen gas; and storing the hydrogen gas in at least one of carbon nanotubes and carbon nanofibers.

[0010] In one embodiment, the electrochemical system comprises: an electrochemical cell comprising a membrane electrode assembly having a hydrogen electrode and a oxygen electrode with a electrolyte disposed therebetween; and hydrogen storage in fluid communication with the hydrogen electrode, wherein the hydrogen storage comprises at least one of carbon nanotubes and carbon nanofibers.

[0011] In another embodiment, the electrochemical system comprises: an electrochemical cell comprising a membrane electrode assembly having a hydrogen electrode and a oxygen electrode with a electrolyte disposed therebetween; and a vessel disposed around the membrane electrode assembly, the vessel defining at least a portion of a hydrogen storage area in fluid communication with the hydrogen electrode and at least a portion of an oxygen storage area in fluid communication with the oxygen electrode; wherein the hydrogen storage area comprises at least one of carbon nanotubes and carbon nanofibers.

[0012] In yet another embodiment, the electrochemical cell system comprises: an electrolysis cell having an electrolysis hydrogen electrode and an electrolysis oxygen electrode with an electrolysis electrolyte membrane disposed therebetween; a fuel cell having a fuel cell hydrogen electrode and a fuel cell oxygen electrode with a fuel cell electrolyte membrane disposed therebetween; and a hydrogen storage tank in fluid communication with the electrolysis hydrogen electrode and the fuel cell hydrogen electrode, the hydrogen storage tank comprises at least one of carbon nanotubes and carbon nanofibers.

[0013] In an embodiment for operating the electrochemical system comprises: introducing oxygen to an oxygen electrode; removing hydrogen from at least one of carbon nanotubes and carbon nanofibers; introducing the hydrogen to a hydrogen electrode; ionizing the hydrogen to produce hydrogen ions and electrons; and combining the oxygen, hydrogen ions and electrons to form water.

[0014] The above discussed and other features will be appreciated and understood by those skilled in the art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Referring now to the drawings, which are meant to be exemplary and not limiting, and wherein like elements are numbered alike in the several Figures:

[0016]FIG. 1 is a schematic diagram of a prior art electrochemical cell showing an electrochemical reaction;

[0017]FIG. 2 is a cross sectional view of an one embodiment of an electrochemical system;

[0018]FIG. 3 is a schematic diagram of another embodiment of an electrochemical system; and

[0019]FIG. 3 is an expanded cross-sectional view of one embodiment of a low gravity electrochemical cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] This application relates to an electrochemical cell and an electrochemical cell system. The system has one or more electrochemical cells and, optionally a compartmentalized storage tank that enable the system to be used in a low or zero gravity environment. Upon application of a load or electricity to the system, the system can operate statically, eliminating the need for both external pumps and the external electrical supply needed for the operation of those pumps. As used herein, “low gravity” means a gravity less than that of the surface gravity of the earth, i.e., less than about 9.78 m/s² (meters per square second), and preferably from about zero to about 9.5 m/s², more preferably from about 0.1 to about 9.0 m/s². Of course, it is to be understood that while useful in low gravity environments, the system may also be used at normal gravity.

[0021] It should be noted that although the system will be described in relation to a proton exchange membrane electrochemical cell employing hydrogen, oxygen, and water, it is readily understood that all types of electrochemical cells can be employed. Additionally, all types of electrolytes may be used, including, but not limited to the following: phosphoric acid, solid oxide, and potassium hydroxide, and the like. Various reactants can also be used, including, but not limited to, the following: hydrogen bromine, oxygen, air, chlorine, and iodine. Upon the application of different reactants and/or different electrolytes, the flows and reactions are understood to change accordingly, as is commonly understood in relation to that particular type of electrochemical cell.

[0022] In one embodiment, the electrochemical cell system can have electrochemical cell(s) disposed within a vessel. Each electrochemical cell including a membrane electrode assembly having a first electrode, a second electrode, and a membrane disposed between, with the vessel disposed around the membrane electrode assembly. This optional vessel can define at least a portion of a first storage area that is in fluid communication with the first electrode and at least a portion of a second storage area that is in fluid communication with the second electrode, wherein, one or both of the storage areas may further comprise at least a portion of one or both flow fields that may surround the electrodes of the membrane electrode assembly.

[0023] Referring to FIG. 2, an electrochemical cell system 33 includes a vessel 31 that encloses an electrochemical cell stack 130. The cell stack 130 comprises one or multiple electrochemical cells and is maintained in position within the vessel 31 using devices such as tie rods, truss rods, bolts, or other devices 34. Each electrochemical cell comprises a MEA 36, with each MEA 36 comprising an oxygen electrode 58 and a hydrogen electrode 60 with a proton exchange membrane (electrolyte) 62 disposed therebetween. The vessel 31 may optionally comprise external ports (not shown) for fluid storage tanks (not shown; see FIG. 3). The external vessels allow for longer operation and/or greater storage capacity.

[0024] The system storage areas 66, 68 preferably comprise a sufficient capacity to hold the desired amount of fluids for the given application. For example, the storage areas can optionally hold the maximum amount of fluid that will be produced during the electrolysis operation. Hydrogen and oxygen are stored in their respective system storage areas 68, 66, within the vessel 31, and optionally in the cell storage areas 70, 72, and/or external to the vessel. Optionally, the hydrogen produced hereby can be stored as high-pressure gas, or alternatively, in a solid form, such as a metal hydride, a carbon based storage (e.g. particulates, nanofibers, nanotubes, or the like), or others, and combinations comprising at least one of the foregoing storage. The use of solid hydrogen storage allows for a reduction in the system storage area 68, which thereby allows for an overall reduction in the size of the electrochemical system 33.

[0025] Referring to FIG. 3, an electrochemical cell 30 comprises a hydrogen flow field 32, an oxygen flow field 34, an MEA 36 disposed therebetween, a water flow field 38, and an electrically conductive porous media divider 40. A compartmentalized storage tank 42 encloses an oxygen storage section 44 and a water storage section 46, which are separated by a movable divider 48. In this embodiment, the oxygen storage section 44 is connected to the oxygen flow field 34 via line 50, the water storage section 46 is connected to the water flow field 38 via line 52, and a hydrogen storage tank 54 is connected to the hydrogen flow field 32 via line 56.

[0026] Storage tank 54 and optional compartmentalized storage tank 42 can be made out of any material that can withstand the required pressures and the exposure to reactants, and preferably comprise a sufficient capacity to hold the desired amount of fluids for the given application. Lines 50, 52, and 56 can also be any material that can withstand the required pressures and the exposure to reactants. The movable divider 48, which separates the oxygen storage section 44, i.e., one fluid section, from the water storage section 46, i.e., another fluid section, comprises any material that is compatible with the stored gas and in any shape that allows the volume of the oxygen storage section 44 to change inversely with the volume of the water storage section 46, while effectively preventing the mixing of the two stored fluids. Possible compartmentalized storage tanks 42 include bladder tanks, bellows tanks, piston tanks, and diaphragm tanks, among others, with a stainless steel vessel having an elastomeric bladder typically preferred for low or zero gravity applications.

[0027] During operation in electrolyzer mode, water is converted to hydrogen gas and oxygen gas. The hydrogen produced hereby can be stored as high pressure gas, or alternatively, in a solid form, such as a metal hydride, a carbon based storage (e.g. particulates, nanofibers, nanotubes, or the like), or others, and combinations comprising at least one of the foregoing storage mediums.

[0028] As oxygen gas is produced, it passes through line 50 to oxygen storage section 44, increasing the pressure of the oxygen therein and placing pressure on the movable divider 48, or the oxygen is vented. The pressure on the movable divider 48 increases the pressure of the water storage section 46, and forces the water through line 52 to the water flow field 38 of electrochemical cell 30.

[0029] In this embodiment, the oxygen pressure is greater than the water pressure, with the difference between the oxygen pressure and the water pressure being below the bubble pressure of the porous media divider 40. In fuel cell mode the operation of the movable divider 48 reverses as stored oxygen converts into water and the pressure in the oxygen storage section 44 decreases. In this mode the water generated by the reaction flows back to the water storage section 46.

[0030] Turning now to FIG. 4, which illustrates an electrochemical cell 30 with more detail, the MEA 36 comprises an oxygen electrode 58, a hydrogen electrode 60, and a proton exchange membrane (electrolyte) 62 disposed therebetween. The membrane 62 can be of any material employed for forming the membrane in electrochemical cells. The electrolytes are preferably solids or gels under the operating conditions of the electrochemical cell. Typically, the thickness of the membrane 62 is up to about 0.05 inches (1.27 millimeters, “mm”), with about 0.001 (0.0254 mm) to about 0.015 inches (0.381 mm) preferred. Useful materials include proton conducting ionomers and ion exchange resins. Proton conducting ionomers comprise complexes of an alkali metal, alkali earth metal salt, or a protonic acid with one or more polar polymers such as a polyether, polyester, or polyimide, or complexes of an alkali metal, alkali earth metal salt, or a protonic acid with a network or crosslinked polymer containing the above polar polymer as a segment. Some useful polyethers include polyoxyalkylenes, such as polyethylene glycol, polyethylene glycol monoether, polyethylene glycol diether, polypropylene glycol, polypropylene glycol monoether, and polypropylene glycol diether, and the like; copolymers of at least one of these polyethers, such as poly(oxyethylene-co-oxypropylene) glycol, poly(oxyethylene-co-oxypropylene) glycol monoether, and poly(oxyethylene-co-oxypropylene) glycol diether, and the like; condensation products of ethylenediamine with the above polyoxyalkylenes; esters, such as phosphoric acid esters, aliphatic carboxylic acid esters or aromatic carboxylic acid esters of the above polyoxyalkylenes. Copolymers of, e.g., polyethylene glycol with dialkylsiloxanes, polyethylene glycol with maleic anhydride, or polyethylene glycol monoethyl ether with methacrylic acid exhibit sufficient ionic conductivity to be useful. Useful complex-forming reagents can include alkali metal salts, alkali metal earth salts, and protonic acids and protonic acid salts. Counterions useful in the above salts can be halogen ion, perchloric ion, thiocyanate ion, trifluoromethane sulfonic ion, borofluoric ion, and the like. Representative examples of such salts include, lithium fluoride, sodium iodide, lithium iodide, lithium perchlorate, sodium thiocyanate, lithium trifluoromethane sulfonate, lithium borofluoride, lithium hexafluorophosphate, phosphoric acid, sulfuric acid, trifluoromethane sulfonic acid, tetrafluoroethylene sulfonic acid, hexafluorobutane sulfonic acid, and the like, as well as combinations comprising at least one of the foregoing salts.

[0031] Ion-exchange resins useful as proton conducting materials include hydrocarbon- and fluorocarbon-type resins. Hydrocarbon-type ion-exchange resins can include phenolic or sulfonic acid-type resins; condensation resins such as phenol-formaldehyde, polystyrene, styrene-divinyl benzene copolymers, styrene-butadiene copolymers, styrene-divinylbenzene-vinylchloride terpolymers, and the like, that are imbued with cation-exchange ability by sulfonation, or are imbued with anion-exchange ability by chloromethylation followed by conversion to the corresponding quaternary amine.

[0032] Fluorocarbon-type ion-exchange resins can include hydrates of a tetrafluoroethylene-perfluorosulfonyl ethoxyvinyl ether or tetrafluoroethylene-hydroxylated (perfluoro vinyl ether) copolymers. When oxidation and/or acid resistance is desirable, for instance, at the cathode of a fuel cell, fluorocarbon-type resins having sulfonic, carboxylic and/or phosphoric acid functionality are preferred. Fluorocarbon-type resins typically exhibit excellent resistance to oxidation by halogen, strong acids, and bases. One family of fluorocarbon-type resins having sulfonic acid group functionality is the NAFION® resins (DuPont Chemicals, Wilmington, Del.).

[0033] The electrodes 58, 60 can be various electrochemical cell electrodes comprising materials such as platinum, palladium, rhodium, iridium, ruthenium, osmium, carbon, gold, tantalum, tin, indium, nickel, tungsten, manganese, and the like, as well as oxides, alloys, and combinations comprising at least one of the foregoing materials. Additional possible electrode catalysts that can be used alone or in combination with the above, include graphite and organometallics, such as pthalocyanines and porphyrins, and combinations thereof, and the like. Some possible catalysts are disclosed in U.S. Pat. Nos. 3,992,271, 4,039,409, 4,209,591, 4,707,229, and 4,457,824, which are incorporated herein by reference. This electrode catalyst can comprise discrete catalyst particles, hydrated ionomer solids, fluorocarbon, other binder materials, other materials utilized with electrochemical cell catalysts, and combinations comprising at least one of the foregoing. Useful ionomer solids can be any swollen (i.e., partially disassociated polymeric material) proton and water conducting material. Possible ionomer solids include those having a hydrocarbon backbone, and perfluoroionomers, such as perfluorosulfonate ionomers (which have a fluorocarbon backbone). Ionomer solids and catalysts therewith are further described in U.S. Pat. No. 5,470,448 to Molter et al., which is incorporated herein by reference.

[0034] In order to allow transport of the electrons, the electrodes electrically connect to an electrical load and/or power source (e.g., 41 in FIG. 3). The electrical connection can comprise any electrical connector such as wires, a truss/bus rod, bus bars, and the like, as well as combinations comprising at least one of these connectors.

[0035] The oxygen flow field 34 and the hydrogen flow field 32 are located on either side of the MEA 36, and typically comprise material that is porous and also electrically conductive. The porous, electrically conductive material is capable of providing structural integrity for supporting the membrane assembly, allowing passage of system fluids to and from the appropriate electrodes 58 or 60, and conducting electrical current to and from the appropriate electrodes 58 or 60.

[0036] The oxygen flow field 34 may comprise a porous material 64 that is disposed adjacent to and in intimate contact with the MEA 36. This optional porous material 64 enables dual-directional flow of gas and water, promoting water flow to and from the membrane by acting as a wick to remove liquid water from the MEA 36, and serves as an electrode support. Optionally, screen(s) 68 are disposed adjacent to the porous material 64, and a flow field member (e.g., one or more screen layers, bipolar plates, or other type of support structures; not shown) may be disposed between the porous material 64 and the electrode 58.

[0037] The water flow field 38, which is disposed adjacent to and in electrical contact with the optional, electrically conductive porous media divider 40, can comprise a similar or different porous conductive material as the other flow fields 32, 34. The material used in the water flow field 38 should allow the flow of water to and from the porous media divider 40.

[0038] The flow fields 32, 34, 38 may each comprise support structures such as screen packs, bipolar plates with grooves or other flow features formed therein, other type of support structures, or combinations of at least one of the foregoing support structures. Suitable screen packs comprise electrically conductive material, such as woven metal, expanded metal, perforated or porous plates, fabrics (woven and non-woven), ceramic (e.g., particulate filled ceramic), polymers or other material, or a combination thereof, which provide structural integrity to the membrane assembly while forming an appropriate flow field for the various fluids and establishing an electron transport to and from the electrodes.

[0039] Typically the screen packs are composed of material such as niobium, zirconium, tantalum, titanium, steels such as stainless steel, nickel, and cobalt, among others, as well as mixtures, oxides, and alloys comprising at least one of the foregoing materials. The geometry of the openings in the screens can range from ovals, circles, and hexagons to diamonds and other elongated and multi-sided shapes. The particular porous conductive material employed is dependent upon the particular operating conditions on that side of the membrane assembly. The size and geometry of the flow fields 32, 34, 38 should be sufficient to enable the desired fluid control. Typically, the thickness is up to about 10 inches (254 mm), with about 0.001 (0.0254 mm) to about 0.1 (2.54 mm) inches preferred.

[0040] The optional porous media divider 40 is typically disposed between and in electrical contact with the oxygen flow field 34 and the water flow field 38. The porous media divider 40 can be any material that provides a barrier to oxygen passage and enables the flow of water between the oxygen flow field 34 and the water flow field 38, while also allowing electron flow to and from the electrodes 58, 60, as described further herein.

[0041] In an alternative embodiment, the optional media divider 40 can be disposed in electrical contact with the hydrogen flow field 32. In this embodiment, water is either produced on the hydrogen electrode and moved through the media divider 40 to the compartmentalized tank 42, or from the compartmentalized tank 42 to the hydrogen electrode 60, through the membrane 62, to the oxygen electrode 58 where it reacts to form oxygen and hydrogen ions.

[0042] The porous media divider 40 can comprise any electrically conductive material compatible with the electrochemical cell environment (for example, the desired pressure differential, preferably up to or exceeding about 4,000 psi, temperatures up to about 250° C., and exposure to hydrogen, oxygen, and water). Some possible materials include carbon, nickel and nickel alloys (e.g., Hastelloy®, which is commercially available from Haynes Ienternational, Kokomo, Ind., and Inconel®, which is commercially available from INCO Alloys International Inc., Huntington, W.Va., among others), cobalt and cobalt alloys (e.g., MP35N®, which is commercially available from Maryland Specialty Wire, Inc., Rye, N.Y., Haynes 25, which is commercially available from Haynes International, and Elgiloy®, which is commercially available from Elgiloy® Limited Partnership, Elgin, Ill., among others), titanium, zirconium, niobium, tungsten, carbon, hafnium, iron (e.g., steels, such as stainless steel, and the like), among others, as well as oxides, mixtures, and alloys comprising at least one of the foregoing materials.

[0043] The particular form of the porous media divider 40 includes forms such as fibrous (random, woven, non-woven, chopped, continuous, and the like), granular, particulate powder, preform, and the combinations comprising at least one of the foregoing forms. The size and geometry of the optional media divider 40 should be sufficient to enable the desired movement of water while inhibiting the flow of oxygen therethrough (or other appropriate fluid control). Typically, the thickness is up to about 0.5 inches (12.7 mm), with about 0.001 inches (0.0254 mm) to about 0.3 inches (7.62 mm) preferred, about 0.001 inches (0.0254 mm) to about 0.05 (1.27) inches more preferred, and about 0.001 inches (0.0254 mm) to about 0.03 inches (0.762 mm) most preferred. Furthermore, to attain the desired flow through the media divider 40, the bubble pressure should be greater than about 0.01 pounds per square inch (psi), with about 7.0 to about 8.0 psi preferred.

[0044] In some embodiments, the electrochemical system can be operated as a fuel cell, an electrolysis cell, or both. Prior to operation as a fuel cell, the stored gas in the compartmentalized storage tank is preferably at a pressure greater than that of the stored water (e.g., up to about 1 psi greater, with about 0.3 psi to about 0.5 psi greater typically sufficient, with a pressure differential up to the bubble pressure of media divider 40 possible). This pressure differential causes the movable divider 48 to be initially biased toward the water storage section 46 of the compartmentalized storage tank 42, forcing water toward the water flow field 38. For example, a mechanical, hydraulic or similar force bias can be employed in the storage tank 42 to push the bellows open, with the bias preferably set at a pressure below the bubble pressure of media divider 40.

[0045] In order to utilize the electrochemical cell 30, it is preferably initially charged by an external power source (not shown). During charging, the cell 30 is operated as an electrolyzer, and water (or other liquid reactant such as hydrogen bromine) is separated, e.g. into hydrogen and oxygen. The hydrogen and oxygen are stored in their respective areas within the system (e.g., hydrogen in storage tank 54 and oxygen in oxygen storage section 44), and, after reaching operating pressure, the system can be used as a fuel cell to produce electricity. That is, the external power source can be disconnected and an electrical load can be attached (e.g., as in load 21 depicted in FIG. 1) to the charged system. The cell 30 can then operate as a fuel cell, recombining the hydrogen and oxygen into water, while producing an electrical current. When no more current is produced, the system is regenerated by again charging with an external power source such as a photovoltaic cell or other power source.

[0046] The stored oxygen from the oxygen storage section 44 and hydrogen from the hydrogen storage tank 54 move to the respective active areas of the electrochemical cell 30. The hydrogen gas flows through the hydrogen flow field 32 and dissociates into hydrogen ions and free electrons at the hydrogen electrode 60. The hydrogen ions move across the proton exchange membrane 62 to the oxygen flow field 34 while the electrons travel through an external load to the oxygen electrode 58. At the oxygen electrode 58, the hydrogen ions combine with oxygen and electrons to form water. The newly formed water moves through the oxygen flow field 34 and is wicked away by the porous media divider 40 to the water flow field 38. From the water flow field 38, the water is piped into the water storage section 46 of the compartmentalized storage tank 42, and the movable divider 48 shifts toward the gas storage side 44 to accommodate the increased water volume.

[0047] When the pressure of the stored gases falls below the level needed to operate the fuel cell 30, an external power supply can be employed. The cell 30 is then operated in reverse, as an electrolyzer, and the stored water, which is biased to a pressure below that of the oxygen gas, is fed into the water flow field 38. The porous media divider 40 wicks the water from the water flow field to the oxygen flow field 34. Not intending to be limited by theory, it is believed that the water, in the form of water vapor, moves through the oxygen flow field 34 to the oxygen electrode 58 where it forms oxygen gas, hydrogen ions, and electrons. Again, the hydrogen ions pass through the membrane 62 and the electrons move through the external load, to the hydrogen electrode 60, where they combine to form hydrogen.

EXAMPLE

[0048] An electrochemical cell having the configuration of cell 30 in FIG. 4 was constructed and tested. The cell had an active area of 0.05 squared feet (46.4 squared centimeters), and was fluidly coupled with a 316 stainless steel water tank having a volumetric capacity of 50 cubed centimeters (cc), a 316 stainless steel hydrogen tank having a volumetric capacity of 500 cc, and a 316 stainless steel oxygen tank having a volumetric capacity of 500 cc. A single cell was constructed using NAFION® membrane, titanium alloy screen packs for the hydrogen flow field, oxygen flow field, and water flow field, and a titanium alloy porous plate for the media divider. The system was operated under zero gravity conditions at 119° F. (48.3° C.), with fluid flows equivalent to pressures of 40 psi and 50 psi for the hydrogen and oxygen, respectively. The following tables summarize polarization data for the above-described cell, wherein Table 1 represents electrolysis data and Table 2 represents fuel cell data. TABLE 1 Electrolysis Current Density (Amperes Current (Amperes) per Squared Feet) Voltage (Volts) 0 0 1.3 5.3 15 1.48 7.1 142 1.66 10.2 204 1.76 15.8 316 1.9 24.7 494 2.17

[0049] TABLE 2 Fuel Cell Current Density (Amperes Current (Amperes) per Squared Feet) Voltage (Volts) 0 0 1.006 0.51 10.2 0.888 1 20 0.831 1.25 25 0.805 1.51 30.2 0.78 1.99 39.8 0.737 2.25 45 0.718 3 60 0.672 3.7 74 0.612 5.4 108 0.543

[0050] The electrochemical cell 30 can be operated in low gravity or even zero gravity environments, such as extraterrestrial environments, without the need for an external electrical supply that would typically be used in a system to power pumps, fans, and other supporting equipment. Cell 30 is effective with or without the compartmentalized storage tank. The electrochemical cell system may be charged in electrolyzer mode by photovoltaic cells attached to the outside of an orbiting space station, for example, and used as a fuel cell to provide electricity thereafter. Since the system is regenerative and does not require the use of pumps, it can serve as a stand-alone system for electrical needs. Additionally, the electrochemical cell system has increased reliability relative to other systems because it has fewer moving components. The system can operate in both low gravity and zero gravity environments, as the need arises. Finally, since the present system uses fewer parts and integrates others, it is more compact than other electrochemical cell systems.

[0051] While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

What is claimed is:
 1. A method for operating an electrochemical system, comprising: introducing water to an oxygen electrode; electrolyzing the water to form oxygen, hydrogen ions and electrons, wherein the hydrogen ions migrate to a hydrogen electrode; reacting the hydrogen ions with the electrons to form hydrogen gas; and storing the hydrogen gas in at least one of carbon nanotubes and carbon nanofibers.
 2. An electrochemical system, comprising: an electrochemical cell comprising a membrane electrode assembly having a hydrogen electrode and a oxygen electrode with a electrolyte disposed therebetween; and hydrogen storage in fluid communication with the hydrogen electrode, wherein the hydrogen storage comprises at least one of carbon nanotubes and carbon nanofibers.
 3. The electrochemical system as in claim 2, further comprising a vessel disposed around the membrane electrode assembly, the vessel defining at least a portion of the hydrogen storage in fluid communication with the hydrogen electrode and at least a portion of an oxygen storage area in fluid communication with the oxygen electrode.
 4. The electrochemical system as in claim 3, wherein the hydrogen storage further comprises a hydrogen storage tank disposed in fluid communication with the vessel and the hydrogen electrode, wherein the hydrogen storage tank comprises at least one of carbon nanotubes and carbon nanofibers.
 5. The electrochemical system as in claim 2, further comprising a compartmentalized storage tank having a first fluid storage section and a second fluid storage section separated by a movable divider, wherein the compartmentalized storage tank is in fluid communication with the electrochemical cell.
 6. An electrochemical system, comprising: an electrochemical cell comprising a membrane electrode assembly having a hydrogen electrode and a oxygen electrode with a electrolyte disposed therebetween; and a vessel disposed around the membrane electrode assembly, the vessel defining at least a portion of a hydrogen storage area in fluid communication with the hydrogen electrode and at least a portion of an oxygen storage area in fluid communication with the oxygen electrode; wherein the hydrogen storage area comprises at least one of carbon nanotubes and carbon nanofibers.
 7. An electrochemical cell system, comprising: an electrolysis cell having an electrolysis hydrogen electrode and an electrolysis oxygen electrode with an electrolysis electrolyte membrane disposed therebetween; a fuel cell having a fuel cell hydrogen electrode and a fuel cell oxygen electrode with a fuel cell electrolyte membrane disposed therebetween; and a hydrogen storage tank in fluid communication with the electrolysis hydrogen electrode and the fuel cell hydrogen electrode, the hydrogen storage tank comprises at least one of carbon nanotubes and carbon nanofibers.
 8. A method for operating an electrochemical system, comprising: introducing oxygen to an oxygen electrode; removing hydrogen from at least one of carbon nanotubes and carbon nanofibers; introducing the hydrogen to a hydrogen electrode; ionizing the hydrogen to produce hydrogen ions and electrons; and combining the oxygen, hydrogen ions and electrons to form water. 